Ink systems for ink-jet printing, of the continuous jet method, in which a steady stream of droplets are being generated, electrostatically charged and then as needed are deflected, or as in a drop-on-demand method, where individual droplets are generated, are becoming more common in many areas.
The requirements on inks used in ink-jet printing methods are strict. Especially they must not clog the ink-jet nozzles. They cannot corrode or attack the printhead materials and must have a good shelf-life, i.e. they should not fail during storage and must retain a constant viscosity and surface tension. Such inks are known.
German Offenstellung 31 06 208 is an ink for ink-jet printing consisting of an aqueous solution of a water-soluble dye and an isomeric alcohol with 1 to 4 carbon atoms, which has a viscosity of 4 to 20 mPas, a surface tension of 40 to 60 mN/m and a specific resistivity of 5.times.10.sup.3 to 1.times.10.sup.5 Ohm.cm. Isomeric alcohols with 1 to 4 carbon atoms are for example glycols. As surfactants, i.e. as a wetting agent one proposes here non-ionic surfactants, such as polyoxyalkylene and its alkylether, siloxane/oxyalkylene-mixing polymer and fluoride containing surface active agents.
Though this ink satisfies the general requirements for ink-jet printing, it has unsatisfactory print quality on plain paper. It quickly fatigues the human eye because the print suffers from irregular fuzzy dot edges. It is therefore unsatisfactory.
In U.S. Pat. No. 4,383,859 an ink for ink-jet printing is described with the following constituents: 3 to 10 wt % dye, 10 wt % Carbowax 200 (polyethylene glycol), 10 wt % diethylene glycol, 4 wt % butyl carbitol, 4 wt % n-methyl-2- pyrrolidone, 0.25 wt % Surfynol 104 (tetramethyl-decyne-diol), 0.2 wt % Triton QS-44 (surfactant), 0.5 wt % Versenol 120, 0.1 wt % Sodium Omadine (a biocide), 64.95 to 67.95 wt % distilled water and 0.1 to 1 wt % ammonium hydroxide.
By and large, this ink is sufficient for the requirements of physical qualities of an ink for ink-jet printing. But in this case as well, plain paper print quality is unsatisfactory because of non-round and fuzzy dot edges. Additionally such inks dry in uncapped nozzles in a few hours.
In U.S. Pat. No. 4,184,881, an ink-jet printing ink is described which deliberately omits the use of glycols or ethylene glycols, and which has in it the following constituents: an aqueous solution of a water soluble dye, together with an ethylene oxide adduct that contains at least an acetylene diol (e.g. 2,4,7,9- tetramethyl-5-decyne-4,7-diol).
This ink also exhibits the necessary physical qualities for ink-jet printing, but is also suffers from lack of print quality on plain paper because of unsatisfactory dot roundness.
Additionally, it has been shown that an uncapped printhead with such ink will be completely plugged after 3 days and then will not be print-ready.
Although ink-jet printing is quite common today, this method of printing on plain paper still poses many problems for the human eye, that is, it tires quickly. Additionally, one could not expect a lot of aesthetic qualities out of such print. Both are consequences of the characteristics of known inks which lead to inadequate roundness of the dots. As a solution to this, special papers are being offered, which result in round dots.